CFD modelling of a water-jet cleaning process for concentrated solar thermal (CST) systems

Abstract

Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.

In this paper we have undertaken an in-depth analysis of water-jet cleaning process for concentrated solar thermal (CST) systems by fluid dynamics simulations. The heliostat surface cleaning efficiency as a function of machine parameters such as nozzle diameter, jet impingement angle, standoff distance, water velocity and nozzle pressure has been modelled with ANSYS CFX software. The scope is to develop an optimized cleaning procedure suitable for CST plants through the correlation between main technical parameters, as described above, and the generated shear stresses on the heliostats surface. In this analysis, shear forces represent the “critical phenomena” at the bottom of soil removal process. Enhancing shear forces on a particular area of the target surface, varying the angle of impingement in combination with the variation of standoff distances, can increase cleaning efficiency. This procedure intends to improve the cleaning operation for CST mirrors reducing spotted surface and increasing particles removal efficiency. The cleaning process is related to the soil removal by erosion resulting from droplets impingement on the surface. It consists of four mechanism types: direct deformation, stress wave propagation, lateral outflow jetting and hydraulic penetration. The first two are responsible for crack initiation in the erosion process as reported by many researchers. The air entrainment process promotes the water-jet spreading followed by a decay of pressure which becomes more as the standoff distance increases with a subsequent reduction in the jet cleaning ability. To keep the jet cleaning ability on the nozzle axis, a standoff distance range has been considered during the cleaning process with a water velocity rage of 80-200 m/s, typically compliant in such cleaning operations. By ANSYS CFX module we have modelled a stationary water-jet system with a single nozzle setup that impinges the jet perpendicular to the flat surface. Several simulations have been carried out varying standoff distance, jet pressure and jet impingement angle in order to identify effective and efficient cleaning procedures to restore heliostats reflectance and CST plant efficiency. Experiments with an array of three nozzles in line are considered in this study in order to evaluate the interaction of the two outermost nozzles with the middle one.